Nowadays monomode optical fibers present very good optical characteristics, with attenuation practically coincident with the lowest limits intrinsically possible for the forming materials, in correspondence with the second transmission window, placed at about 1.3 .mu.m wavelength. At this wavelength, monomode fibers having a simple refractive-index profile, of the step-index or depressed-index cladding type, present null chromatic dispersion and attenuation values of about 0.3 dB/km.
Thus, transmission systems operating at high transmission rates, e.g. 2 Gbit/s, may be realized with fiber trunks longer than 100 km and without intermediate repeaters. By way of example the memorandum entitled: "A 130 km transmission experiment at 2 Gb/s using silica-core fiber and a vapour phase transported DFB laser" by B. L. Kasper et al., contained in X.ECOC, Stuttgard 1984 PD-6 is worth mentioning.
Optimization of transmissive properties of silica, the material the optical fibers are made from, results more complex in correspondence with the third transmission window, placed at about 1.55 .mu.m. In fact, at such wavelengths fibers present the lowest attenuation values (less than 0.2 dB/km), but at the same time their chromatic dispersion values are about 20 ps/nm.multidot.km.
The latter value highly limits the system bandwidth.
To overcome this disadvantage monomode fibers having guide structures with more complex refractive-index profiles (W, triangular, multiplecore) are being designed and tested.
With these structures systems operating with about 200 km repeater spacings and high transmission rate may be possibly implemented, even though difficulties in the simultaneous optimization of fibre attenuation and bandwidth, as a function of the refractive-index profile complexity, might arise.
The possibility exists of lengthening the repeater spacing, by increasing the transmission capacity by the use of coherent-type transmission systems. Notwithstanding, not to decrease the minimum detectable signal level, the output polarization state must be well defined and to correspond to that of the local oscillator. Which can be achieved by using either highly-birefringent fibers, such as to transmit a single-polarization state in the fundamental mode, or fibers having a reduced birefringent degree, but allowing a low coupling between the two polarization states of the fundamental mode.
Highly-birefringent fibers have already been proposed and fabricated: among them elliptic core, elliptic cladding, panda, and bowtie fibers are worth mentioning. In the elliptic-core type birefringence is induced by creating geometric asymmetry, i.e. the core presents an elliptic section propagating the radiation with a polarization plane parallel to the major ellipse axis. In the other fibers birefringence is generated by asymmetric stress. The method consists in applying a pressure to the core along a preferential direction. As a consequence the core refractive index undergoes a variation in the force direction, and hence the core develops to a high degreee, birefringence characteristics.
Nevertheless, both geometric and mechanical asymmetry introduce perturbations into the material structure, which perturbations affect the most critical fiber zone, i.e. the core-cladding interface.
Such perturbation causes a residual attenuation according to a phenomenon mostly apparent at high wavelengths, i.e. in the region where optical fibers present the lowest attenuation values.
The use of the third transmission window (1.55 .mu.m) is indispensable for coherent-type transmission systems, both because at these wavelengths there are the lowest attenuation values, and because the conversion of the modal polarization state due to the natural Rayleigh scattering phenomenon (which is present when trunk lengths of the order of 100 km are considered) is less important at high wavelengths.
Said disadvantages are overcome by the method of fabricating birefringent optical fibers provided by the present invention, which allows the fabrication of monomode fibers with a certain degree of birefringence but without mechanical asymmetry or induced geometric asimmetry.
Besides, such a method allows the use of established methods of optical fiber preform fabrication. Thus considerable length fiber-trunks with homogeneous optical characteristics and not so liable to be affected by external perturbations may be produced.